A conventional parabolic antenna weather radar forms a thin beam called a pencil beam and horizontally rotates it through 360° to acquire observed data of one plane. Then, the elevation angle of the antenna is increased to acquire the next plane. This operation is continued, thereby collecting three-dimensional precipitation data (for example, “Radar Techniques”, revised edition compiled under the supervision of Takashi Yoshida, IEICE, Oct. 1, 1996, first edition, chap. 3, Meteorological Radar, pp. 238-239). Executing this observation sequence requires 5 to 10 minutes. It is therefore impossible to obtain sufficient time resolution and spatial resolution for observation of, for example, cumulonimbus clouds which change from hour to hour.